Dehydroxylation and purification of calcium fluoride materials using a halogen containing plasma

Abstract

The invention is directed to a process of purifying metal fluoride materials used to make metal fluoride single crystals suitable for making optical elements used in the transmission of wavelengths below 200 nm, and in particular to a process of purifying such materials by the use of a halogen containing plasma to convert metal oxygenates contaminating the feedstocks used in the preparation of the crystals to metal fluorides. The invention also is directed to a process of growing a metal fluoride single crystal using a crystal growth furnace to carry out the foregoing purification procedure followed by the steps of melting the purified material and cooling it using s selected time and temperature cycle to from a metal fluoride single crystal. The plasmas used in practicing the invention can be derived from a variety of halogenated materials including, for example, fluorocarbons, chlorocarbons, boron trihalides, chlorine, fluorine, xenon difluoride and other gaseous or easily volatilized halogenated substances known in the art.

Description

FIELD OF THE INVENTION [0001] The invention is directed to a process of purifying metal fluoride materials used to make metal fluoride single crystals suitable for making optical elements used in the transmission of wavelengths below 200 nm, and in particular to a process of purifying such materials by the use of a halogen containing plasma to convert metal oxygenates contaminating the feedstocks used in the preparation of the crystals to metal fluorides. BACKGROUND OF THE INVENTION [0002] The burden of the demands for improved performance of computers and other electronic devices falls on the lithographic processes used to fabricate integrated circuit chips. Lithography involves irradiating a mask and focusing the pattern of this mask through an optical microlithography system onto a wafer coated with a photoresist. The pattern on the mask is thereby transferred onto the wafer. Decreasing the line-widths of the features on a given wafer enables the writing on a wafer of more elemen...

AI Technical Summary

Problems solved by technology

The burden of the demands for improved performance of computers and other electronic devices falls on the lithographic processes used to fabricate integrated circuit chips.

However, the commercial use and adoption of below 200 nm wavelengths has been hindered by the transmission nature of such deep ultraviolet wavelengths through optical materials and by the lack of economically manufacturable, high quality blanks of optically transmissive materials suitable for below 200 nm microlithography optical elements.

Commercially available raw material powders are sufficiently pure in terms of cationic impurities, but these materials are not sufficiently pure in terms of the anionic impurities they contain.

While the use of an oxygen scavenger during the crystal growth process can alleviate the oxygen contamination problem, such use can give rise to additional problems.

This is frequently difficult to determine.

While the use of excess scavenger does not present problems when the scavenger is gaseous, when a solid scavenger such as PbF2 is used, the excess of scavenger could result in lead contamination of the grown crystal.

If insufficient scavenger is used, then residual oxygen containing substances may be present in the grown crystal and these substances will absorb radiation in the 120-220 nm regions, and consequently interfere with the transmission properties of the crystal and optical element made from it.

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